1. Field of the Invention
The invention relates to a hydroelastic deep-drawing apparatus in presses for the drawing of sheet-metal shapes, and more particularly to a hydroelastic deep-drawing apparatus having a ram, a punch, a plurality of short-stroke cylinders, a plurality of drawing pins, a sheet holder plate, a drawing-die, a platen, a plurality of short-stroke pistons, a die cushion-plate, and a differential cylinder/piston system.
2. Description of the Related Art
The object of German Patent No. 4,100,206 is that disadvantages of prior art deep-drawing devices are avoided while bringing about a computer-assisted process-controllable hydroelastic deep-drawing. The apparatus of the above-mentioned German patent has drawing pins, arranged according to the geometric shape of the workpiece and according to the hole pattern, which are activated in the sheet-holder plate action zone and the drawing-punch action zone such that an exact force-profile control, without disturbing amplitudes, is obtained, thereby achieving the advantages and effects of an elastomeric die-cushion system.
In the solution of the above-identified patent, the following is described:
a) a die-cushion plate which has a multi-point control according to the geometric shape of the workpieces provided, and within the die-cushion plate target points, corresponding to a grid spacing or hole pattern having the dimensions "a" and "b", are assigned a hydraulic short-stroke piston with attachable drawing pins, the stroke of the short-stroke piston being no more than 10 mm, PA0 b) each short-stroke piston is assigned a short-stroke cylinder with selectively connectable hydraulic force action zones according to the workpiece or the sheet-holder plate, PA0 c) the die-cushion plate is supported hydraulically at each of the four outer corners by a differential cylinder/piston system of regulated position and parallel run, and PA0 d) by an appropriate control of the multi-point position, and regulation of the differential cylinder/piston system, eccentric forces, occurring in the event of a changing surface force center of gravity in the force action zones, are automatically absorbed hydraulically. PA0 that the lubricating-film texture applied to the sheet on both sides is damaged or even destroyed, and PA0 that impact markings, caused by the dynamic shock, leave their trace on the sheet and, even where shapes undergo less deep-drawing, as for example flat body parts, the impact markings persist as a marking on the deep-drawn shapes thereby requiring additional finishing measures, as for example the application of coats of enamel, so that they can be applied to finished parts-by. PA0 a) from the beginning to the end of the deep-drawing operation, to execute an exact process control such that the sheet-holding forces are to be controlled in the region of the drawing-die geometry "proportionally to the desired value", that is to say, without force peaks in the control trend over the deep-drawing stroke, PA0 b) during the deep-drawing operation, the sheet-holding forces can be regulated from P=0 to P=max. (kN), without an interruption of the ram movement, and PA0 c) the impact speed is reduced until, in order to avoid uncontrolled masses and therefore force effects, the possibility of a passive nonpositive connection between the ram and drawing appliance by way of the workpiece is reliably prevented.
The measures and features according to the invention make it possible, in particular, for a plurality of action zones of the sheet-holder plate and drawing punch, to achieve the advantage associated with the method of an elastomeric sheet-holder/die-cushion system, but also with the advantage and possibility of using a computer-assisted process control. The solution according to the aforementioned patent makes it possible to activate a plurality of hydraulic cylinders in a very confined space in a process-controllable and computer-assisted manner. That is to say, in an identically repeatable manner using the same program, without each individual cylinder having to be equipped with separately controlled valves.
Where deep-drawing presses are concerned, it is especially noticeable that a sharp and uncontrolled impact shock of the ram on the deep-drawing sheet (workpiece) has the following disadvantages:
According to the state of the art, in all drawing appliances, with or without force action zones, or even in four-point drawing systems, a passive non-positive connection with the sheet-holder plate is always obtained by the displacement of hydraulic oil which is counter to a pressurized valve.
In order to minimize the impact shock on the workpiece, European patent specification 0,074,421 discloses a technical solution in which the sheet-holder plate is preaccelerated to approximately a speed which is synchronous with the ram, by means of a servo-hydraulic device and a movement-cycle control integrated therein. Using this system, it is not possible to prevent a considerable residual impact from adversely acting on the workpiece.
It must be mentioned, as a disadvantage of the device and control according to European patent specification 0,074,421, that additional measures are needed to reduce the mass-dynamic impact shock. To be more precise, there has to be an additional hydraulic actuator which preaccelerates the total mass of the sheet-holder, together with the drawing appliance, by means of a very complex servo-hydraulic crank control. This entails the disadvantage that, at the moment of contact with the sheet, the ram, together with the drawing-die plate, must overtake the sheet-holder plate. As before, the total mass of the drawing appliance has to be accelerated according to the differential speed .DELTA.V. This dynamic "residual impact" will always be too high. In all events, these forces are equal to or higher than the hydraulic pressure surges which occur in an unreliable way in such hydraulic drawing appliances.